Beirut — A Syrian rebel bomb attack reduced the army headquarters in Damascus to a smouldering wreck on Wednesday as world leaders, unable to break the diplomatic deadlock in the conflict, met at the United Nations.

The army said four guards were killed and 14 wounded in what it said were suicide attacks. No senior officers were hurt in the blasts, which shook the whole city just before the start of the working day, it said.

Since then Assad's forces have pushed back rebels to the outskirts of the capital but have lost control of several border crossings, struggled to win back the northern city of Aleppo and mounted air strikes to crush opposition in rebel territory.

State television showed CCTV footage of a white minibus pulling up by the side of the road and exploding in a ball of flames. It showed another blast 10 minutes later, apparently inside the complex.

The explosions struck as world leaders met at the United Nations, where deadlock over Syria has blocked a united global response to a conflict which activists say has killed 30,000 people, forced a quarter of a million refugees to flee the country and left 2.5 million people in need of help.

The uprising, which erupted in March last year as mainly peaceful protests for reform, has become an armed insurgency pitting mainly Sunni Muslim rebels against Assad, from the Alawite faith which is close to Shi'ite Islam.

One Sunni leader, the Emir of Qatar, told the United Nations that Arab countries should intervene "to stop the bloodshed", but few Arab states are likely to back his call.

In Moscow, Russian President Vladimir Putin rebutted calls for an intervention. Any attempt to unilaterally use force or interfere with events in the Middle East would be counter-productive, he said.

The pro-opposition Syrian Observatory for Human Rights, which monitors the violence through a network of activists in the country, said 240 people were killed in Syria on Tuesday. Most were civilians but the death toll included 54 members of Assad's security forces.

Activists said security forces killed more than 40 people in a town outside Damascus on Thursday, calling it a massacre.

Video published by activists showed rows of bloodied corpses wrapped in blankets in the town of Dhiyabia. The victims appeared to be male, from 20-year-olds to elderly men.

The Syrian Observatory said it could confirm 40 dead.

"A massacre in the Dhiyabia area," says the voice of an activist in the video. "God damn you Bashar. The bodies are in the dozens. Look, Muslims, look what this dictator is doing."

Flames engulf military building

Internet footage of Wednesday's fire at the General Staff Command Building showed flames engulfing its upper floors.

"The attack in Damascus once again proves that, with sufficient planning and co-ordination, the opposition appears to retain the ability to strike at the heart of regime," said David Hartwell, Middle East analyst at IHS Jane's.

"This is despite the fact that the FSA has appeared in recent weeks to be under pressure as a result of the fighting in Aleppo and other parts of the country."

The main gate of the military complex was blackened from fire while windows of the building were blown out. Glass shards littered streets and a deep crater was gouged in the road.

Residents reported that gunfire rattled out around the district for at least two hours after the explosions.

"All our colleagues in the military leadership, the army staff command and the Defence Ministry are unhurt," Information Minister Omran Zoabi told Syrian Television.

"It's a terrorist act, close to an important site, that's true. But as usual they failed to achieve their goal," he said.

Activist Samir al-Shami said the main explosions were caused by a suicide car bomb and second car loaded with explosives on the perimeter of the complex.

"Then the fighters went inside and clashed with security inside, while some of the men started to torch the building," he said.

That tallied with accounts from residents who heard gunfire and smaller blasts after the first explosions.

"The explosions were very loud. They shook the whole city and the windows of our house were shuddering," one resident reached by telephone said.

A correspondent for Iran's English-language Press TV was shot dead by a rebel sniper and its Damascus bureau chief was wounded while they covered Wednesday's explosions, Press TV said.

Pro-Assad gunmen also killed at least 16 people in Damascus, the British-based Observatory said. It said three of those killed in the poor district of Barzeh, which is sympathetic to opposition fighters, were children and six were women.

No future

"The Syrian regime ...has no future among us," Hollande said in a speech on Tuesday. "Without any delay, I call upon the United Nations to provide immediately to the Syrian people all the support it asks of us and to protect liberated zones."

Protection for "liberated" areas would require no-fly zones enforced by foreign aircraft, which could stop deadly air raids by Assad's forces on populated areas. But there is little chance of securing a Security Council mandate for such action given the opposition of veto-wielding members Russia and China.

The United States, European allies, Turkey and Gulf Arab states have sided with the Syrian opposition while Iran, Russia and China have backed Assad, whose family and minority Alawite sect have dominated Syria for 42 years.

Western powers have stopped short of supplying military aid to the rebels to an extent that could turn the tide of the conflict, in part out of fear of arming Islamist militants who have joined the anti-Assad revolt.